This project involves the study of a series of porphyrins which, in the presence of light, catalyze production of a potent cytotoxic agent, singlet oxygen. Using appropriate animal models and cell-culture systems, we plan to investigate structure-activity relations, among the porphyrins, study determinants of the selective toxicity of these agents, and use different poryphins, varying in lipid solubility as probes for different membrane regions. Since the toxic action of the poryphins appears to be localized at or near the surface, effects of these photo-activated drugs on immunologic properties of affected cells will also be assessed.